Why: The approved r0.5 design needs an auditable zero-paid execution sequence after the failed r0.4 model sample.
Evidence: The approved design object is Documents 0d473fe; current source seams, file ownership, relative links, and whitespace were checked before drafting this plan.
Impact: Add five test-first work units and explicit stage, budget, privacy, and stop gates. This commit contains no product implementation or model call.
Refs: U3 r0.5 design; Dev task feature-fast-slow-u2-u3-plan
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